Ocular optics system having at least four reflections occurring between curved surfaces

ABSTRACT

The invention relates to an ocular optics system which forms no intermediate image, is reduced in terms of size and weight with well-corrected aberrations, and is best suited for use on face- or head-mounted image display devices, and provides an ocular optics system 7 comprising three juxtaposed optical surfaces 3, 4 and 5, wherein a space defined by three optical surfaces 3, 4, and 5 is filled with a transparent medium having a refractive index greater than 1, at least two optical surfaces 3 and 4 of these three optical surfaces are defined by curved surfaces concave on a pupil position side of the optics system, and at least four reflections occur between curved surfaces 3 and 4.

BACKGROUND OF THE INVENTION

The present invention relates generally to an ocular optical system, andmore particularly to an ocular optical system intended for use with ahead- or face-mounted image display device which is mounted on the heador face of the user to project images into the user's eyeballs.

In recent years, helmet or goggle-type of head-mounted or face-mountedimage display devices have been developed for virtual reality purposesor with a view to allowing individuals to enjoy wide-screen viewing.

For instance, JP-A-2-297516 discloses an optical system made up of a 2Ddisplay device 11 for displaying images, an objective collimating lens12 and a parallel transparent plate 13 having off-axis paraboloidalmirrors on both its ends, as shown in FIG. 6. Light beams leaving the 2Ddisplay device 11 for displaying images are converted by the objectivecollimating lens 12 into parallel beams, which are then subjected tofirst transmission through a parallel surface of the paralleltransparent plate 13, reflection at the first paraboloidal mirror, sometotal reflections within the parallel transparent plate 13, reflectionat the second paraboloidal mirror and second transmission through theparallel surface of the parallel transparent plate 13 (8 reflections and2 transmissions), whereby an intermediate image is formed on a point Ffor projection into an observer's eyeball 14.

U.S. Pat. No. 4,026,641 discloses an optical system wherein, as shown inFIG. 7, a image of an image display element 11 is converted by atransmission optical element 15 into a curved object image, which is inturn projected from a toric surface 16 into an observer's eyeball 14.

A problem associated with an image display device of the type wherein animage thereof is relayed as shown in FIG. 6 is, however, that it needsnot only an ocular optical system, but also a relay optical system,resulting in increases in the size and weight of the whole opticalsystem as well as an increase in the amount of extension of the wholeoptical system from a person's head or face. Therefore it is not fit fora head-mounted or face-mounted image display device.

The optical system for forming parallel beams as an intermediate imageas well as the optical system for projecting an intermediate image intoan eyeball produce some considerable aberrations because only theparaboloidal mirrors that have power.

The ocular optical system composed of a concave mirror, as shown in FIG.7, also produces some considerable aberrations and so is detrimental toimage quality, even though the concave mirror is defined by a toricsurface as shown in FIG. 7.

To correct for field curvature occurring at the ocular optical system,therefore, it is required to use the transmission optical element 15such as a fiber plate. Even by use of the transmission optical system 15and toric surface 16, however, it is impossible to make adequatecorrection for coma, and other aberrations.

SUMMARY OF THE INVENTION

In view of such problems associated with the prior art as mentionedabove, an object of the present invention is to provide an ocularoptical system designed to form no intermediate image, especially, anocular optical system which is compact and light in weight with wellcorrected aberrations, and so is best suited for use on a head- orface-mounted image display device.

To achieve the aforesaid object, the present invention provides anocular optical system comprising at least three juxtaposed opticalsurfaces, characterized in that at least two optical surfaces of said atleast three optical surfaces are defined by curved surfaces which areconcave on a pupil position side of said optical system and at leastfour reflections occur between said curved surfaces.

In this case, it is desired that a space defined by said at least threesurfaces be filled with a transparent medium having a refractive indexgreater than 1.

It is also desired that an image display element be located on an objectsurface of said ocular optical system, and that a display surface ofsaid image display element be directed to a side opposite to said pupilposition.

An account will now be given of why the aforesaid arrangement is used inthe present invention and how it works.

The present invention is concerned with an optical system layout neededfor locating an ocular optics system in a compact manner. In otherwords, making the ocular optical system is important for making, forinstance, an image display device on which it is used thin. By makingthe display device thin, it is possible to reduce the moment of inertiaeven at the same weight because the center of gravity comes close to thecenter of an observer's head. In short, the ability of the displaydevice to follow the movement of the observer's head is much improved.

According to the present invention, therefore, there is provided anocular optical system designed to project an image of an image displayelement directly into an observer's eyeball without recourse to anyrelay optical system.

According to the present invention, the ocular optical system cansuccessfully be made thin by allowing light rays to reciprocate therein,thereby defining a turn-back optics path.

It should here be noted that only by use of a turn-back optics path itis impossible to achieve a wide-enough viewing angle. For this reason,it is essentially important that at least two reflecting surfaces aredefined by curved surfaces concave on the pupil position side of theoptical system, and that light rays reflect between the curved surfaceswhile a light beam converges simultaneously with repeated reflectionwithin the ocular optical system.

In other words, reflecting surfaces of concave mirrors, convex mirrorsor the like are more reduced than refracting surfaces having the samepower in terms of the quantity of aberrations generated, and do not giverise to chromatic aberration at all. The use of three or more reflectingsurfaces having power enables the power to be so dispersed thatprojection can be achieved with more reduced aberrations if the power ison the same level. By properly locating concave and convex mirrors atproper intervals it is additionally possible to keep aberrations in agood state because field curvature, spherical aberration and otheraberrations generated at the reflecting surfaces of those concave andconvex mirrors can be offset with one another.

If there is provided a turn-back optics path allowing at least fourreflections to occur between those mirrors, it is then possible to makethe optical system compact and thin.

If two of at least three optical surfaces are located with their concavesurfaces directed to the observer's eyeball located on the pupilposition side of the optical system, it is then possible to obtain, withreduced coma, an observed image which has high resolution yet is clearover its whole length and breadth.

If the space defined by the aforesaid three surfaces is filled with amedium having a refractive index greater than 1, it is then possible toconstruct the reflecting surfaces of back-surface mirrors, and hence,reduce the occurrence of coma and spherical aberration. This is becauseback-tracing light rays from the pupil converge upon being transmittedthrough the second transmitting surface (a second transmitting surfaceas counted from the image display element side) whereby the diversion oflight rays within the optical system is more reduced as compared with anoptical system defined by surface mirrors when making sure of the sameviewing angle, so that it is possible to reduce aberrations generated atthe reflecting surfaces and, at the same time, to make the opticalsystem compact without giving rise to any shading of light rays.

When the ocular optical system of this invention is used with an imagedisplay device, it is desired that the image display element be locatedon the object surface of the ocular optics system with its displaysurface located on the opposite side of the pupil position. It is thenpossible to reduce the proximal-to-distal thickness of the opticalsystem from the pupil in the optical axis direction. This in turn makesit possible to reduce the amount of extension of the optical system fromthe observer's face upon being built in an image display device.

If at least three optical surfaces are all arranged with their concavesurfaces located on the pupil position side of the optical system, it isthen possible to obtain, with much more reduced coma, an observed imagewhich has high resolution yet is clear over its whole length andbreadth.

Further, it is desired that an optical axis emerging from the objectsurface of the ocular optics system progresses in the order of a firsttransmitting surface, a first reflecting surface, a second reflectingsurface, a third reflecting surface, a fourth reflecting surface, afifth reflecting and a second transmitting surface. This ensures that atleast five reflections occur at the first to fifth reflecting surfaceswhile the optical axis emerges from the second transmitting surface viathe first transmitting surface, so making it possible to construct allthe reflecting surfaces of back-surface mirrors and, at the same time,to turn back the optical path reciprocally.

If the curvatures of at least three optical surfaces within theirdecenteration planes and within planes perpendicular thereto arechanged, it is then possible to correct for astigmatism occurring atdecentered concave mirrors.

If the first transmitting surface and the second reflecting surface aremade up of surfaces of the same shape at the same location, it is thenpossible to facilitate the fabrication of the ocular optical systembecause the number of surface shapes needed therefor is reduced.

Similarly, if the fourth reflecting surface and the second transmittingsurface are made up of surfaces of the same shape at the same location,it is then possible to facilitate the fabrication of the ocular opticssystem because the number of surface shapes needed therefor is reduced.

If the second and fourth reflecting surfaces are made up of surfaces ofthe same shape at the same location, it is then possible to make thefabrication of the ocular optical system easier because the number ofsurface shapes needed therefor is reduced.

If the first transmitting surface, the second and fourth reflectingsurfaces and the second transmitting surface are made up of surfaces ofthe same shape at the same location, it is then possible to make thefabrication of the ocular optical system easier because the number ofsurface shapes needed therefor is much more reduced.

If the first and third reflecting surfaces are made up of surfaces ofthe same shape at the same location, it is then possible to make thefabrication of the ocular optical system easier because the number ofsurface shapes needed therefor is much more reduced.

If the first transmitting surface, the second and fourth reflectingsurfaces and the second transmitting surface are made up of surfaces ofthe same shape at the same location, and if the first and thirdreflecting surfaces are made up of surfaces of the same shape at thesame location, it is then possible to facilitate make the fabrication ofthe ocular optical system because the number of surface shapes neededtherefor is reduced.

If an image display element is located on the object surface of theocular optical system with a locating means provided to locate theobserver's eyeball on the pupil position, it is then possible toconstruct an image display device of small size.

If a locating means is provided to locate the image display element andthe ocular optical system on an observer's head so that they can bemounted together on the observer's head, it is then possible for theobserver to view images in a free position or direction. In short, theobserver can view images in a comfortable position. For instance, even abedridden, ill person can view images in a bedridden position if theimage display device is worn on the person's head. It is thus possibleto construct a head-mounted image display device of small size.

If an image pickup means is located on the object surface of the ocularoptical system with a stop positioned at the pupil position, it is thenpossible to provide an image pickup optical system of small size, and bythe addition thereto of function capable of forming an image of anobject at infinity, it is further possible to provide an image opticalsystem such as a finder optical system used on video cameras or thelike.

Of course, in this case, the ocular optical system of this invention isusable as part of microscopes, telescopes, endoscopes, and so on.

By constructing the second reflecting surface of a convex mirror concaveon the observer's eyeball side, it is further possible to cancel fieldcurvature and other aberrations generated at the first and thirdreflecting surfaces, and so reduce the quantity of aberrations generatedover the ocular optical system.

By constructing the first, third and fifth reflecting surfaces ofconcave mirrors concave on the pupil side and the second and fourthreflecting mirrors of convex mirrors concave on the pupil side, it isfurther possible to achieve apositive/negative/positive/negative/positive power layout favorable forcorrecting for coma, field curvature and other aberrations.

If the second reflecting surface is designed such that light rays areincident thereon at an angle of incidence exceeding the critical angle,the light rays are then reflected at the second reflecting surface inthe form of total reflection so that a reflectance of 100% can obtained.This in turn enables light loss in an observed image to be reduced andmake the observed image bright. This also enables the reflecting andtransmitting regions to be superposed on each other so that the opticalsystem can be made to be compact.

It is particularly preferable to construct the optical system with asingle optical element because the optical system can have threeactions, viz., “first transmitting action upon incidence on the opticalelement”, “five reflections within the optical element”, and “secondtransmitting action upon emanating from the optical element”. Thus, theuse of a single optical element can simplify the fabrication of theocular optical system.

If the second and fourth reflecting mirrors are designed such that lightrays are incident thereon at an angle of incidence exceeding thecritical angle, the light rays are then reflected at the second andfourth reflection surfaces in the form of total reflection. This in turnenables light loss in an observed image to be reduced and make theobserved image bright. This also enables the reflecting and transmittingregions to be superposed on each other so that the optical system can bemade compact.

Of importance in this invention is that the ocular optical systemsatisfies the following condition (1):

70°<θ₂<100°  (1)

with the proviso that a light ray leaving the center of an object pointand arriving at the center of the pupil defines a principal ray. Here θ₂represents an angle which the principal ray incident on the secondreflecting surface makes with the principal ray emerging therefrom. Thiscondition is provided to determine the longitudinal size of the opticalsystem. At an angle smaller than the lower-limit angle of 70°, the firsttransmitting surface and second reflecting surface of the optical systeminterfere with each other, so rendering it impossible to make sure ofany wide viewing angle. At an angle exceeding the upper-limit angle of100° it is difficult to make the optical system compact because itslongitudinal size becomes large.

Of importance in this invention is also that the ocular optics systemsatisfies the following condition (2):

110°<θ₄<160°  (2)

wherein θ₄ represents an angle which the principal ray incident on thefourth reflecting surface makes with the principal ray emergingtherefrom. This condition is again provided to determine thelongitudinal size of the optical system. At an angle smaller than thelower-limit angle of 110°, the first transmitting surface and fourthreflecting surface of the optical system interfere with each other, sorendering it impossible to make sure of any wide viewing angle. At anangle exceeding the upper-limit angle of 160° it is difficult to makethe optical system compact because its longitudinal size becomes large.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrative of Example 1 of an image displaydevice making use of the ocular optical system according to the presentinvention.

FIG. 2 is a sectional view illustrative of Example 2 of the imagedisplay device making use of the ocular optics system according to thepresent invention.

FIG. 3 is a sectional view illustrative of Example 3 of the imagedisplay device making use of the ocular optical system according to thepresent invention.

FIG. 4 is a schematic illustrative of one exemplary general constructionof a portable type of image display device making use of the ocularoptical systems according to the present invention.

FIG. 5 is a sectional view illustrative of one optical system shown inFIG. 4.

FIG. 6 is a schematic illustrative of one conventional head-mountedimage display device, and

FIG. 7 is a schematic illustrative of another conventional head-mounteddisplay device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples 1 to 3 of the image display device constructed using the ocularoptics system according to the present invention will now be explainedwith reference to the accompanying drawings.

Constitutional parameters of each example will now be given later. Inwhat follows, surface numbers are given by back-tracing surfaces numbersas counted from an observer's pupil position 1 toward an image displayelement 6. As shown in FIG. 1, a coordinate system is composed of theorigin defined by an observer's iris position 1, a Z axis defined by anobserver's visual axis 2, whose direction from the origin toward anocular optical system 7 is taken as being positive, a Y axisperpendicular to the observer's visual axis 2, whose direction frombelow to above with respect to an observer's eyeball is taken as beingpositive, and an X axis perpendicular to the observer's visual axis 2,whose direction from right to left with respect to the observer'seyeball is taken as being positive. In other words, a Y-Z plane isdefined within a sheet plane of FIG. 1 while an X-Z plane is defined bya plane perpendicular to the sheet plane. An optical axis is hereassumed to be turned back within the Y-Z plane on the sheet plane.

Of the constitutional parameters to be described later, Y, Z and θrepresent amounts of decenteration of a vertex of a given surface fromthe reference surface (pupil position 1) in the Y and Z axis directionsand an angle of inclination of a center axis of that given surface fromthe Z axis, respectively. Note that the plus sign attached to θ meansthe direction of counterclockwise rotation, and that the surfaceseparation is of no significance.

For each surface, an irrotationally symmetric aspheric shape is given by

Z=[(X ² Rx)+(Y ² /Ry)Y[1+{1−(1+Kx)(X ² /Rx ²)−(1+Ky)(Y ² /Ry ²)}^(½)]+AR[(1−AP)X ²+(1+AP)Y ²]² +Br[(1−BP)X ²+(1+BP)Y ²]³

where, on the coordinates defining that surface, Ry and Rx are theparaxial radii of curvature of the aspheric surface within the Y-Z plane(sheet plane) and the X-Z plane, respectively, Kx and Ky are the conicalcoefficients of the aspheric surface within the X-Z plane and the Y-Zplane, respectively, AR and BR are the fourth- and sixth-order asphericcoefficients of the aspheric surface which is rotationally symmetricwith respect to the Z axis, respectively, and AP and BP are the fourth-and sixth-order aspheric coefficients of the aspheric surface which isirrotationally symmetric with respect to the Z axis, respectively.

It is here to be noted that the refractive index of a medium betweensurfaces is given by a d-line refractive index, and that length is givenin units of millimeters.

In the constitutional parameters to be described later, back-tracing iscarried out from an object point position of a virtual image positioned1-meter away from the pupil.

FIGS. 1, 2 and 3 are sectional views of Examples 1, 2 and 3 of amonocular image display device. In these drawings, reference numeral 1stands for an observer's pupil position, 7 an ocular optical system, 2an observer's visual axis, 3 a first surface of the ocular opticalsystem 7, 4 a second surface of the ocular optical system 7, 5 a thirdsurface of the ocular optical system, and 6 an image display element.

Referring to actual optical paths in these examples, a pencil of lightemanating from the image display element 6 is first incident on theocular optical system 7 upon being refracted at the first surface 3thereof, then is internally reflected at the second surface 4, the firstsurface 3, the second surface 4, the first surface 3 and the thirdsurface 5 in the described order, then incident on the first surface 3where it is refracted, and finally projected into an observer's eyeballwhile the iris position of the observer's pupil or the center ofrotation of the observer's eyeball is taken as the exit pupil 1.

Horizontal and vertical field angles, and pupil diameter are 18.75° and25°, and 8 mm for Example 1; 22.5° and 30°, and 4 mm for Example 2; and30° and 22.5°, and 4 mm for Example 3.

Throughout the examples, the surfaces are all anamorphic asphericsurfaces, and the first transmitting surface, second reflecting surface,fourth reflecting surface and second transmitting surface comprise thecommon first surface 3 while the first reflecting surface and thirdreflecting surface comprise the common second surface 4.

It should be noted that the ocular optical system of this invention isusable as an image optical system capable of forming an image of anobject point at a distant place.

Enumerated below are the values of the constitutional parameters inExamples 1, 2 and 3. It is to be noted, however, that θ₁ to θ₅ representangles which, as shown in FIG. 2, the principal ray—defined by a lightray leaving the center of the image display element 6 and arriving atthe center of the pupil 1—incident on the first to fifth reflectingsurfaces makes with the principal ray emerging therefrom.

EXAMPLE 1

Sur- face Sur- sep- Refractive face Radius of ar- index Abbe's No. No.curvature ation (Displacement) (Tilt angle) 1 ∞ (pupil) 2 R_(y) −427.5181.51633 64.1 R_(x) −29.212 Y 17.473 θ −5.351° K_(y) 0 Z 26.387 K_(x) 0AR 0 BR 0 AP 0 BP 0 3 R_(y) −254.887 1.51633 64.1 R_(x) −649.767 Y130.176 θ 12.443° K_(y) 0 Z 52.819 K_(x) 0 AR −0.876797 × 10⁻⁶ BR 0 AP−0.713380 BP 0 4 R_(y) −427.518 1.51633 64.1 R_(x) −29.212 Y 17.473 θ−5.351° K_(y) 0 Z 26.387 K_(x) 0 AR 0 BR 0 AP 0 BP 0 5 R_(y) −242.5041.51633 64.1 R_(x) −42.527 Y −31.772 θ −8.869° K_(y) 0 Z 36.510 K_(x) 0AR 0 BR 0 AP 0 BP 0 6 R_(y) −427.518 1.51633 64.1 R_(x) −29.212 Y17.473 θ −5.351° K_(y) 0 Z 26.387 K_(x) 0 AR 0 BR 0 AP 0 BP 0 7 R_(y)−242.504 1.51633 64.1 R_(x) −42.527 Y −31.772 θ −8.869° K_(y) 0 Z 36.510K_(x) 0 AR 0 BR 0 AP 0 BP 0 8 R_(y) −427.518 Y 17.473 θ −5.351° R_(x)−29.212 Z 26.387 K_(y) 0 K_(x) 0 AR 0 BR 0 AP 0 BP 0 9 (display device)Y 55.708 θ −22.615° Z 21.266 θ₁ = 45° θ₂ = 76° θ₃ = 94° θ₄ = 128° θ₅ =65°

EXAMPLE 2

Sur- face Refractive Sur- sep- index face Radius of ar- (Displace-Abbe's No. No. curvature ation ment) (Tilt angle) 1 ∞ (pupil) 2 R_(y)−152.846 1.51633 64.1 R_(x) −66.891 Y −13.768 θ −13.449° K_(y) 0 Z27.500 K_(x) 0 AR −0.178178 × 10⁻⁸ BR −0.594362 × 10⁻¹¹ AP 0.105146 × 10BP 0.664504 3 R_(y) −119.363 1.51633 64.1 R_(x) −82.534 Y 47.793 θ6.177° K_(y) 0 Z 54.768 K_(x) 0 AR −0.190871 × 10⁻⁶ BR 0.305496 × 10⁻¹¹AP 0.660695 × 10⁻¹ BP 0.487236 4 R_(y) −152.846 1.51633 64.1 R_(x)−66.891 Y −13.768 θ −13.419° K_(y) 0 Z 27.500 K_(x) 0 AR −0.178178 ×10⁻⁸ BR −0.594362 × 10⁻¹¹ AP 0.105146 × 10 BP 0.664504 5 R_(y) −118.6261.51633 64.1 R_(x) −82.339 Y −26.271 θ −14.237° K_(y) 0 Z 40.718 K_(x) 0AR −0.122336 × 10⁻⁷ BR −0.514035 × 10⁻¹² AP −0.236341 × 10 BP 0.831265 6R_(y) −152.846 1.51633 64.1 R_(x) −66.891 Y −13.768 θ −13.419° K_(y) 0 Z27.500 K_(x) 0 AR −0.178178 × 10⁻⁸ BR −0.594362 × 10⁻¹¹ AP 0.105146 × 10BP 0.664504 7 R_(y) −118.626 1.51633 64.1 R_(x) −82.339 Y −26.271 θ−14.237° K_(y) 0 Z 40.718 K_(x) 0 AR −0.122336 × 10⁻⁷ BR −0.514035 ×10⁻¹² AP −0.263241 × 10 BP 0.831265 8 R_(y) −152.846 Y −13.768 θ−13.419° R_(x) −66.891 Z 27.500 K_(y) 0 K_(x) 0 AR −0.178178 × 10⁻⁸ BR−0.594362 × 10⁻¹¹ AP 0.105146 × 10 BP 0.664504 9 (display device) Y45.606 θ −1.625° Z 22.237 θ₁ = 52° θ₂ = 85° θ₃ = 87° θ₄ = 124° θ₅ = 66°

EXAMPLE 3

Sur- face Refractive Sur- sep- index face Radius of ar- (Displace-Abbe's No. No. curvature ation ment) (Tilt angle) 1 ∞ (pupil) 2 R_(y)−95.092 1.51633 64.1 R_(x) −36.857 Y 36.645 θ −1.558° K_(y) 0 Z 36.258K_(x) 0 AR 0.939620 × 10⁻⁸ BR 0.455727 × 10⁻¹⁷ AP −0.110527 × 10⁻¹ BP−0.168168 × 10⁻¹² 3 R_(y) −106.440 1.51633 64.1 R_(x) −60.074 Y 27.597 θ−22.699° K_(y) 0 Z 55.143 K_(x) 0 AR −0.405140 × 10⁻⁶ BR 0.141026 ×10⁻¹⁸ AP 0.318736 × 10⁻² BP 0.285034 × 10⁻¹² 4 R_(y) −95.092 1.5163364.1 R_(x) −36.857 Y 36.645 θ 1.558° K_(y) 0 Z 36.258 K_(x) 0 AR0.939620 × 10⁻⁸ BR 0.455727 × 10⁻¹⁷ AP −0.110527 × 10⁻¹ BP −0.168168 ×10⁻¹² 5 R_(y) −84.273 1.51633 64.1 R_(x) −49.707 Y 35.482 θ 11.744°K_(y) 0 Z 43.297 K_(x) 0 AR 0.708627 × 10⁻⁷ BR −0.117003 × 10⁻¹⁵ AP0.721109 × 10⁻¹ BP −0.112555 × 10⁻¹¹ 6 R_(y) −95.092 1.51633 64.1 R_(x)−36.857 Y 36.645 θ 1.558° K_(y) 0 Z 36.258 K_(x) 0 AR 0.939620 × 10⁻⁸ BR0.455727 × 10⁻¹⁷ AP −0.110527 × 10⁻¹ BP −0.168168 × 10⁻¹² 7 R_(y)−84.273 1.51633 64.1 R_(x) −49.707 Y 35.482 θ 11.744° K_(y) 0 Z 43.297K_(x) 0 AR 0.708627 × 10⁻⁷ BR −0.117003 × 10⁻¹⁵ AP 0.721109 × 10⁻¹ BP−0.112555 × 10⁻¹¹ 8 R_(y) −95.092 R_(x) −36.857 Y 36.645 θ 1.558° K_(y)0 Z 36.258 K_(x) 0 AR 0.939620 × 10⁻⁸ BR 0.455727 × 10⁻¹⁷ AP −0.110527 ×10⁻¹ BP −0.168168 × 10⁻¹² 9 (display device) Y 46.315 θ −16.589° Z28.915 θ₁ = 56° θ₂ = 86° θ₃ = 98° θ₄ = 138° θ₅ = 73°

In the aforesaid examples the anamorphic surfaces are used; however, itshould be understood that use may be made of any desired surfacesinclusive of toric surfaces, rotationally symmetric surfaces, sphericalsurfaces, or free curved surfaces represented by the following equation:$Z = {\sum\limits_{n = 0}^{k}\quad {\sum\limits_{m = 0}^{K}\quad {C_{nm}x^{m}y^{n - m}}}}$

where x, y and z represent orthogonal coordinates. C_(nm) is anarbitrary constant, and k and k′ are arbitrary constants.

It should also be understood that use may be made of such holographicsurfaces as set forth in JP-A-7-104209.

For surfaces that cannot be defined in terms of curvature, power, etc.,it is possible to determine their curvature and power by finding thecurvature of a certain region obtained by differentiation of the shapeof surface portion coming in contact with an axial light ray propagatingon the visual axis and arriving at the image display element along thataxial light ray.

Such an ocular optical system according to this invention is used withan image display element in a set-up form. Two such sets are supportedwhile they are spaced away from each other by an interpupillary distanceto thereby construct a portable type of image display device such as afixed or head-mounted type of image display device which one can use toview images with both eyes. It should be understood that one such setmay be used in the form of a monocular image display device. Oneexemplary general construction of such a portable image display deviceis shown in FIG. 4, and a section of one such set intended for use withone eye of an observer is illustrated in FIG. 5. As shown in FIG. 5, adisplay device body 50 includes a pair of two such sets of ocular opticssystems 7 in association of which an image display element 6 comprisingan LCD is located on an image surface. As shown in FIG. 4, the devicebody 50 is provided on both the temples with continuous temple frames51, which are connected to each other via a parietal frame 52. Betweenthe temple frames 51 there is located a rear frame 54 via a leaf spring53. The rear frame 54 is engaged with the rear sites of both the ears ofthe observer, as is the case with the bows of glasses, while the displaydevice body 50 is mounted on the head of the observer, whereby thedisplay device 50 can be well held in front of the eyes of the observer.It is here to be noted that a parietal pad 55 formed of an elasticmaterial such as sponge is contained in the inside of the parietal frame52 and a similar pad is contained in the inside of the rear frame 54 aswell, so that the observer can comfortably wear this display device onhis or her head.

The rear frame 54 is additionally provided with a speaker 56 to enablethe observer to hear stereophonic sounds while viewing images. Thedisplay device body 50, having the speaker 56, may be connected to aplayback 58 such as a portable video cassette via an image/soundtransmission cord 57, so that the observer can wear the playback 58 onany desired position of a belt or the like to enjoy images with sounds.Reference numeral 59 stands for volume switches or other controls of theplayback 58. Note that the parietal frame 52 has built-in electronicparts for image- and sound-processing circuits.

The cord 57 may have a jack at a distal end for connection with anexisting video deck or the like. Further, the cord may be connected witha TV wave reception tuner such that the user may watch television or,alternatively, with a computer to receive computer graphics images ormessage images therefrom. Furthermore, an antenna may be used in placeof such an awkward cord to receive external signals via electromagneticwaves.

The principles, and some examples, of the present invention have beendescribed above; however, it is understood that the present invention isin no sense limited to such examples, and many other modifications maybe possible.

According to the present invention it is possible to achieve an ocularoptical system which, as explained herein at great length, comprises atleast three juxtaposed optical surfaces, at least two optical surfacesof said at least three optical surfaces being defined by curved surfacesconcave on a pupil position side of said optical system and at leastfour reflections occurring between said curved surfaces, and so isunlikely to form any intermediate image, is reduced in terms of size andweight with well-corrected aberrations, and is best suited for use onhead- or face-mounted image display devices as well.

The entirety of JP-A-8-7363 filed Jan. 19, 1996, from which priorityunder 35 USC 119 is claimed, is incorporated herein by reference:

What we claim is:
 1. An ocular optical system comprising: at least threejuxtaposed optical surfaces, at least two optical surfaces of said atleast three optical surfaces are defined by curved surfaces which areconcave on a pupil position side of said ocular optical system and atleast four reflections occur between said curved surfaces, wherein saidat least four reflections are four combined reflections occurring atsaid at least two optical surfaces.
 2. An ocular optical systemcomprising: at least three juxtaposed optical surfaces, at least twooptical surfaces of said at lest three optical surfaces are defined bycurved surfaces which are concave on a pupil position side of saidocular optical system and at least four reflections occur between saidcurved surfaces wherein said ocular optical system satisfies thefollowing condition: 70°<θ₁<100° with the proviso that a light rayleaving a center of an object point and arriving at a center of a pupildefines a principal ray, wherein θ₂ represents an angle which theprincipal ray incident on a second reflecting surface makes with theprincipal ray emerging therefrom, as counted in the order of progress ofthe light ray.
 3. An ocular optical system comprising: at least threejuxtaposed optical surfaces at least two optical surfaces of said atleast three optical surfaces are defined by curved surfaces which areconcave on a pupil position side of said ocular optical system and atleast four reflections occur between said curved surfaces, wherein saidocular optical system satisfies the following condition: 110°<θ₄<160°with the proviso that a light ray leaving a center of an object pointand arriving at a center of a pupil defines a principal ray, wherein θ₄represents an angle which the principal ray incident on a fourthreflecting surface makes with the principal ray emerging therefrom, ascounted in the order of progress of the light ray.
 4. An ocular opticalsystem according to claim 1, wherein that a space defined by said atleast three surfaces is filled with a transparent medium having arefractive index greater than
 1. 5. An ocular optical system accordingto claim 1, 2, or 3 wherein an image display element is located on anobject surface of said ocular optical system, and a display surface ofsaid image display element is directed to a side opposite to said pupilposition.
 6. An ocular optical system according to claim 1, 2 or 3,wherein said at least three optical surfaces are all concave on thepupil position side of said ocular optical system.
 7. An ocular opticalsystem according to claim 1, 2 or 3, wherein an optical axis emergingfrom an object surface of said ocular optical system progresses in anorder of a first transmitting surface, a first reflecting surface, asecond reflecting surface, a third reflecting surface, a fourthreflecting surface, a fifth reflecting surface and a second transmittingsurface.
 8. An ocular optical system according to claim 7, wherein saidfirst transmitting surface and said second reflecting surface areconstructed of surfaces of a substantially same shape at a substantiallysame location.
 9. An ocular optical system according to claim 8, whereinsaid second and fourth reflecting surfaces are constructed of surfacesof a substantially same shape at a substantially same location.
 10. Anocular optical system according to claim 7, wherein said firsttransmitting surface, said second and fourth reflecting surfaces andsaid second transmitting surface are constructed of surfaces of asubstantially same shape at a substantially same location.
 11. An ocularoptical system according to claim 7, wherein said first and thirdreflecting surfaces are constructed of surfaces of a substantially sameshape at a substantially same location.
 12. An ocular optical systemaccording to claim 7, wherein said fourth reflecting surface and saidsecond transmitting surface are constructed of surfaces of asubstantially same shape at a substantially same location.
 13. An ocularoptical system according to claim 12, wherein said second and fourthreflecting surfaces are constructed of surfaces of a substantially sameshape at a substantially same location.
 14. An ocular optical systemaccording to claim 1, 2, or 3 wherein curvatures of said at least threeoptical surfaces within planes of decentration thereof are differentfrom those within planes perpendicular thereto.
 15. An ocular opticalsystem according to claim 6, wherein said first transmitting surface,said second and fourth reflecting surfaces and said second transmittingsurface are constructed of surfaces of a substantially same shape at asubstantially same location, and said first and third reflectingsurfaces are constructed of surfaces of a substantially same shape at asubstantially same location.
 16. An ocular optical system according toclaim 1, 2 or 3, characterized by being usable as an image pickupoptical system by locating including an image pickup means located on anobject surface of said ocular optical system and a stop at a pupilposition of said ocular optical system.
 17. An image display device,comprising: an ocular optical system having at least three juxtaposedoptical surfaces, at least two optical surfaces of said at least threeoptical surfaces are defined by curved surfaces which are concave on apupil position side of said ocular optical system and at least fourreflections occur between said curved surfaces; an image display elementlocated on an object surface of said ocular optical system; and alocating means provided to locate an eyeball of an observer on a pupilposition of said ocular optical system, wherein said at least fourreflections are four combined reflections occurring at said at least twooptical surfaces.
 18. An image display device according to claim 17,wherein said ocular optical system satisfies the following condition:70°<θ₂<100° with the proviso that a light ray leaving a center of anobject point and arriving at a center of a pupil defines a principalray, wherein θ₂ represents an angle which the principal ray incident ona second reflecting surface makes with the principal ray emergingtherefrom, as counted in the order of progress of the light ray.
 19. Animage display device according to claim 17, wherein said ocular opticalsystem satisfies the following condition: 110°<θ₄<160° with the provisothat a light ray leaving a center of an object point and arriving at acenter of a pupil defines a principal ray, wherein θ₄ represents anangle which the principal ray incident on a fourth reflecting surfacemakes with the principal ray emerging therefrom, as counted in the orderof progress of the light ray.
 20. A head-mounted image display device,comprising: an ocular optical system having at least three juxtaposedoptical surfaces, at least two optical surfaces of said at least threeoptical surfaces are defined by curved surfaces which are concave on apupil position side of said ocular optical system and at least fourreflections occur between said curved surfaces; an image display elementlocated on an object surface of said ocular optical system; and alocating means provided to locate an eyeball of an observer on a pupilposition of said ocular optical system, wherein said at least fourreflections are four combined reflections occurring at said at least twooptical surfaces.
 21. A head-mounted image display device according toclaim 20, wherein said ocular optical system satisfies the followingcondition: 70°<θ₂<100° with the proviso that a light ray leaving acenter of an object point and arriving at a center of a pupil defines aprincipal ray, wherein θ₂ represents an angle which the principal rayincident on a second reflecting surface makes with the principal rayemerging therefrom, as counted in the order of progress of the lightray.
 22. A head-mounted image display device according to claim 20,wherein said ocular optical system satisfies the following condition:110°<θ₄<160° with the proviso that a light ray leaving a center of anobject point and arriving at a center of a pupil defines a principalray, wherein θ₄ represents an angle which the principal ray incident ona fourth reflecting surface makes with the principal ray emergingtherefrom, as counted in the order of progress of the light ray.
 23. Anoptical system placed between an image plane and a pupil, said opticalsystem comprising: at least three juxtaposed optical surfaces, at leasttwo optical surfaces of said at least three optical surfaces beingdefined by curved surfaces which are concave on a pupil position side ofsaid optical system and at least four reflections occur between saidcurved surfaces, wherein said at least four reflections are fourcombined reflections occurring at said at least two optical surfaces.24. An optical system placed between an image plane and a pupil, saidoptical system comprising: at least three juxtaposed optical surfaces,at least two optical surfaces of said at least three optical surfacesbeing defined by curved surfaces which are concave on a pupil positionside of said optical system and at least four reflections occur betweensaid curved surfaces, wherein said optical system satisfies thefollowing condition: 70°<θ ₂ <100° with the proviso that a light rayleaving a center of an object point and arriving at a center of thepupil defines a principal ray, wherein θ ₂ represents an angle which theprincipal ray incident on a second reflecting surface makes with theprincipal ray emerging therefrom, as counted in the order of progress ofthe light ray.
 25. An optical system placed between an image plane and apupil, said optical system comprising: at least three juxtaposed opticalsurfaces, at least two optical surfaces of said at least three opticalsurfaces being defined by curved surfaces which are concave on a pupilposition side of said optical system and at least four reflections occurbetween said curved surfaces, wherein said optical system satisfies thefollowing condition: 110°<θ ₄ <160° with the proviso that a light rayleaving a center of an object point and arriving at a center of thepupil defines a principal ray, wherein θ ₄ represents an angle which theprincipal ray incident on a fourth reflecting surface makes with theprincipal ray emerging therefrom, as counted in the order of progress ofthe light ray.
 26. An optical system according to claim 23, wherein aspace defined by said at least three surfaces is filled with atransparent medium having a refractive index greater than
 1. 27. Anoptical system according to claim 23, 24, or 25, wherein said at leastthree optical surfaces are all concave on said pupil position side ofsaid optical system.
 28. An optical system according to claim 23, 24, or25, wherein a first transmitting surface, a first reflecting surface, asecond reflecting surface, a third reflecting surface, a fourthreflecting surface, a fifth reflecting surface and a second transmittingsurface are placed in recited order along an optical path from saidimage plane to said pupil.
 29. An optical system according to claim 28,wherein said first transmitting surface and said second reflectingsurface are constructed of surfaces of a substantially same shape at asubstantially same location.
 30. An optical system according to claim29, wherein said second and fourth reflecting surfaces are constructedof surfaces of a substantially same shape at a substantially samelocation.
 31. An optical system according to claim 28, wherein saidfirst transmitting surface, said second and fourth reflecting surfacesand said second transmitting surface are constructed of surfaces of asubstantially same shape at a substantially same location.
 32. Anoptical system according to claim 28, wherein said first and thirdreflecting surfaces are constructed of surfaces of a substantially sameshape at a substantially same location.
 33. An optical system accordingto claim 28, wherein said fourth reflecting surface and said secondtransmitting surface are constructed of surfaces of a substantially sameshape at a substantially same location.
 34. An optical system accordingto claim 33, wherein said second and fourth reflecting surfaces areconstructed of surfaces of a substantially same shape at a substantiallysame location.
 35. An optical system according to claim 23, 24, or 25,wherein curvatures of said at least three optical surfaces within planesof decentration thereof are different from those within planesperpendicular thereto.
 36. An optical system according to claim 27,wherein said first transmitting surface, said second and fourthreflecting surfaces and said second transmitting surface are constructedof surfaces of a substantially same shape at a substantially samelocation, and said first and third reflecting surfaces are constructedof surfaces of a substantially same shape at a substantially samelocation.
 37. An optical system according to claim 23, 24, or 25, usableas an image pickup optical system and including an image pickup meanslocated on an object surface of said optical system and a stop at apupil position of said optical system.